1. Field of the Invention
This invention relates generally to a vehicle powertrain applicable to a hybrid electric vehicle (HEV). More particularly, the invention pertains to controlling the stopping and restarting of an engine on a road grade.
2. Description of the Prior Art
A HEV is a vehicle that combines a conventional propulsion system, which includes an internal combustion engine and a step-change automatic transmission, a rechargeable energy storage system that includes an electric motor and electric storage battery to improve fuel economy over a conventional vehicle.
Motor vehicles can be designed to employ certain aspects of hybrid electric technology, but without use of a hybrid electric powertrain. Certain vehicles having a conventional powertrain, but no electric machine for driving the wheels, called micro-HEVs, shutdown the engine during conditions where the engine operates at idle speed to reduce fuel consumption and reduce emissions while the vehicle is stopped.
During normal vehicle operation many instances arise where the vehicle must stop: at traffic signals, cross-walks, stop signs and the like. In micro-HEVs the engine is shut down if no power is required, e.g. while waiting at a traffic light. As soon as power is requested, the engine is automatically restarted. By avoiding an unnecessary engine idling event, the vehicle's fuel economy is improved. For this purpose, it is desirable to shut down the engine function as much as possible when certain engine stop conditions are satisfied.
A vehicle stopped on a surface that has a sufficient grade or slope and whose powertrain includes an automatic transmission, can experience a vehicle roll-back event while the engine is idling. A conventional automatic transmission is driven by the engine through a torque converter. With the vehicle on a flat surface and the engine at idle, torque transmitted to the transmission is generally sufficient to enable slight forward rolling of the vehicle, i.e., vehicle creeping. When the vehicle is on a slight grade of positive slope (3%-7%), torque transferred to the transmission is generally sufficient to hold the vehicle stationary preventing roll-back. On higher grades (7% and greater), however, vehicle roll-back can occur, causing reverse torque transfer through the transmission.
In a micro-HEV with the engine shutdown and the vehicle stationary on an uphill road grade, vehicle roll-back and reverse torque transfer effects can be worse because there is no torque output from the engine before the engine startup and insufficient traction torque during engine restart.
During an engine startup in gear process, the gradient load torque TRL is transferred from the wheels by the driveline to the transmission. TRL=mg sin e*Rw, wherein (m) is vehicle mass and (e) is the road gradient angle and Rw is the effective tire radius. TRL is transmitted to the engine as additional load during an engine restart. When the hill gradient is 3% and higher, the following three problems can be observed during micro-hybrid vehicle starts on hill. First, a torque surge caused by the engine restart may bring unexpected vehicle forward motion, which is both undesirable for the vehicle and uncomfortable for the driver. Such a torque spike phenomenon will be more substantial on a descending hill. Second, additional negative torque load on the engine caused by TRL can stall the engine during engine restart, because the initial engine restart torque and the starter may not be large enough to drive the additional load. Third, after the engine restart, the creep torque at engine idle speed may not be sufficient to counteract the road gradient torque load on the vehicle before the driver applies the accelerator pedal. As a result, the vehicle will roll back on the incline before the drive actively depresses the accelerator pedal to power the vehicle.